1. Field of the Invention
The present invention relates to an exposure control apparatus for a camera by means of which the information of the brightness of the object to be photographed and other photographing informations are operated so as to obtain a proper exposure.
2. Description of the Prior Art
Until now, such exposure control apparatus have been widely used that are so designed that, by means of an output of the operation amplifier combined with the output of a photo electric transducing means like SBC or, an ampere meter is controlled. This is done in such a manner that the shutter time or the aperture value is obtained by means of the indication of the ampere meter.
However, in case of such apparatus, the above mentioned operation amplifier is used only as means for amplifying the output of the photo electric transducing means whereby the photographing informations such as the film sensitivity, the shutter time or the aperture value are introduced by rotating the ampere meter itself. Therefore, the composition of such apparatus is unavoidably very complicated.
Until now, in order to overcome such shortcoming as mentioned above, it has been proposed that, as shown in FIG. 1, a bridge circuit consisting of the photo sensitive element 3, the variable resistance 4 for setting the photographing informations and the logarithmically compressing diodes 5 and 6 is connected to the current source 1 through the switch 2. Accordingly the ampere meter 7 is connected to the output terminal of the above mentioned bridge circuit in such a manner that the ampere meter 7 is controlled by means of a current according to the polarity of the unbalance voltage of the above mentioned bridge circuit. Further, it has also been proposed to connect the differential amplifying circuit consisting of the transistors 8 and 9 between the bridge circuit and the ampere meter 7, so as to control the ampere meter 7 as shown in FIG. 2.
Even by means of such apparatus as mentioned above, a complete temperature compensation can be obtained when the current flowing through the right logarithmically compressing diode 5 and that flowing through the left logarithmically compressing diode 6 are equal to each other while depending upon the value of the brightness of the object to be photographed and that of other photographing informations, the value of the current flowing through the diode 5 and that of the current flowing through the diode 6 are different from each other. This current difference is such a difference between the temperature characteristics of the logarithmically compressing diode 5 and that of the logarithmically compressing diode 6 is caused to take place so as to bring about an exposure error. FIG. 3 shows the temperature characteristics of logarithmically compressing diode whereby, along the abscissa, the forward voltage VF and along the ordinate the forward current IF is shown. When the ambient temperature T is increased from T.sub.1 up to T.sub.2, the VF - IF characteristics of the diode is varied as shown in the drawing. Thus, it is understood that depending upon the temperature, the voltage between both terminals of the logarithmically compressing diode varies whereby depending upon its current value, the quantity of the variation is also varied in case a constant current runs through the diode.
The first purpose of the present invention therefore is to obtain the proper exposure by controlling the exposure control circuit, by means of the operation amplifying circuit the first input of which is the information of the brightness of the object to be photographed and the second input of which is other photographing informations.
The second purpose of the present invention is to obtain a complete temperature compensation of the compressed information of the brightness of the object to be photographed and other compressed informations for the whole light measuring range.
The third purpose of the present invention will be disclosed in the course of the explanation to be made below as to the embodiments of the present invention.